This application claims the benefit of Japanese Patent application Nos.11-361353, 2000-069068 and 2000-378695 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a variable focal length lens system and, in particular, relates to a variable focal length lens system with an angle of view of 80xc2x0 or more in the wide-angle end state.
2. Description of Related Art
The primary marketing need for a lens shutter type camera is good portability. The portability is classified into compactness and lightweight. Since the total length of a picture-taking lens affects the size of a camera body and the diameter of a lens affects the height and width of a camera body, to miniaturize a picture-taking lens greatly affects the compactness of a camera body.
It has recently become common to use a zoom lens system in a picture-taking lens used for a lens shutter type camera because a zoom lens system has a merit for a photographer to make it possible to take picture at will in accordance with variation of focal length. Moreover, since a zoom lens system having a longer focal length in a telephoto end state, which gives the longest focal length, makes it possible to take a picture closer to an object, the zooming ratio shows a tendency to become higher, so that the focal length in the telephoto end state becomes longer.
A zoom lens system used for a lens shutter type camera has had a so-called telephoto type power arrangement having a negative lens group to the most image side of the lens system and magnifying an image formed by a lens group arranged to the object side of the negative lens group. In a zoom lens system with high zoom ratio, it has been the mainstream that a so-called multi-group zoom lens having more than three movable lens groups is applied to a zoom lens system in order to increase the zoom ratio.
In one of typical examples of the specific lens constructions of these multi-group zoom lens systems, it has been known a positive-positive-negative three-group type that has, in order from an object side, a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power. Moreover, for another example, it has been known a positive-negative-positive-negative four-group type that has, in order from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a negative lens group.
These zoom lens systems have a negative lens group to the most image side. Therefore, the back focal length in a wide-angle end state, which gives the shortest focal length, becomes short to a certain extent, and off-axis light flux passing through the negative lens group passes away from the optical axis. When the state of lens group positions is changed from the wide-angle end state to the telephoto end state, the negative lens group is moved to the object side such that the interval between an aperture stop and the negative lens group decreases, so that off-axis light flux passing through the negative lens group passes near to the optical axis, and, as a result, variation in off-axis aberrations corresponding to the change in the lens group positions is corrected well.
Furthermore, as for a positive-negative-positive-negative four-group type, zoom lens systems disclosed, for example, in Japanese Laid-Open Patent Application No. 60-57814,and as for a positive-positive-negative three-group type, zoom lens systems disclosed in Japanese Laid-Open Patent Application No. 5-264903 have been known.
Moreover, a current lens shutter type camera has an autofocus function as a standard specification.
An autofocus mechanism is composed of a detector for detecting a position of an object, a calculating system for calculating a driving amount based on an output from the detector, a driver for driving a focusing lens group, and an optical system corrected with its variation of aberrations produced while focusing.
As the autofocus function becomes a standard specification to be equipped, the focusing action is made an attempt to become faster. In order to make the focusing action fast, it is important to reduce workload (=weightxc3x97moving distance).
Moreover, if a time lag, which is a time interval from an instant a shutter button is pressed by a photographer to an instant the actual exposure starts, is too long, the photographer may feel sense of discomfort. Accordingly, the time lag is desirably to be short for a nimble picture taking.
The smaller the moving distance and the size of the focusing lens group are, the faster the focusing action can be. Therefore, in order to make the time lag in the focusing action smaller, it is important to provide an optical system having proper construction.
In the focusing method of a zoom lens system, there have been known the following three methods:
(A) a first lens group focusing type
(B) an inner lens group focusing type
(C) a rear lens group focusing type
where in (A) a first lens group focusing type, the most object side lens group is moved, in (C) a rear lens group focusing type, the most image side lens group is moved, and, in (B) an inner lens group focusing type, an intermediate lens group is moved for focusing, respectively.
By the way, a lens shutter type camera is relatively low price and has good portability and easy handling in comparison with a single lens reflex camera. Accordingly, the user class of a lens shutter type camera has a high percentage of general class who has a camera for the purpose of family photographs and travel photographs, which have real relevance for the daily lives. Accordingly, the position of an object to be photographed tends to be rather near to the photographer.
As described above, a long focal length zoom lens is effective when the position of an object is far away. Therefore, a zoom lens system having short focal length and wide angle of view in the wide-angle end state contrary to the telephoto end state has been developed.
However, when a multi-group type zoom lens according to a prior art is used, it is very difficult to make shorter the focal length of the zoom lens in the wide-angle end state with keeping the compactness of the diameter of the zoom lens in comparison with making it longer in the telephoto end state.
In a zoom lens system with a wide angle of view, off-axis light flux passing through the aperture stop and lens groups tends to pass away from the optical axis. Moreover, when a zoom lens system keeps a wider angle of view, the brightness of the edge of the image field tends to decrease, so that in order to keep the sufficient brightness of the edge of the image field, the diameter of the lens system tends to become large. Therefore, it is not suitable for pursuing compactness.
In a lens system disclosed in Japanese Laid-Open Patent Application No. 60-57814, the angle of view in the wide-angle end state was narrow. In a lens system disclosed in Japanese Laid-Open Patent Application No. 5-264903, compactness could not be fully accomplished.
In a multi-group type zoom lens system such as a positive-positive-negative three-group type or a positive-negative-positive-negative four-group type, the diameter of the lens in the first lens group or the negative lens group located to the most image side is bigger relative to that in the other lens groups. Accordingly, the first lens group focusing method (A) and the rear lens group focusing method (C) in which the lens group having a large lens diameter is moved for focusing is not advantageous for making a camera compact, and so that it has been difficult to make the AF function (focusing movement) fast.
The present invention is made in view of the aforementioned problems and has an object to provide a variable focal length lens system with an angle of view of more than 80xc2x0 in the wide-angle end state.
According to one aspect of the present invention, a variable focal length lens system includes, in order from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a negative refractive power. When the state of lens positions is changed from a wide-angle end state to a telephoto end state, all the lens groups are moved to the object side such that the interval between the first lens group and the second lens group increases, the interval between the second lens group and the third lens group decreases, and the interval between the third lens group and the fourth lens group decreases. Each lens group from the first lens group through the third lens group is composed of two lens elements or more. The following conditional expressions (1) and (2) are satisfied:
3 less than f1/fw less than 6xe2x80x83xe2x80x83(1) 
0.3 less than |f2|/(fwxc2x7ft)xc2xd less than 0.6xe2x80x83xe2x80x83(2) 
where f1 denotes the focal length of the first lens group, fw denotes the focal length of the variable focal length lens system in the wide-angle end state, f2 denotes the focal length of the second lens group, and ft denotes the focal length of the variable focal length lens system in the telephoto end state.
In one preferred example of the present invention, an aperture stop is arranged between the second lens group and the third lens group, and the first lens group includes two lens elements which is a negative lens element having a concave surface facing to the object side and a positive lens element having a convex surface facing to the object side. The following conditional expression (3) is satisfied:
0.05 less than fw/|ra| less than 0.55xe2x80x83xe2x80x83(3) 
where ra denotes the radius of curvature to the object side of the negative lens element in the first lens group.
In one preferred example of the present invention, the second lens group includes a negative lens element located to the most object side of the second lens group and the following conditional expression (4) is satisfied:
0.7 less than fw/|rb| less than 1.3xe2x80x83xe2x80x83(4) 
where rb denotes the radius of curvature to the object side of the negative lens element located to the most object side of the second lens group.
In one preferred example of the present invention, the second lens group is composed of a double concave lens element and a positive lens element having a convex surface facing to the object side and locating to the image side of the double concave lens element. The following conditional expression (5) is satisfied:
0.9 less than (rc+rd)/fw less than 1.6xe2x80x83xe2x80x83(5) 
where rc denotes the radius of curvature to the image side of the double concave lens element in the second lens group, and rd denotes the radius of curvature to the object side of the positive lens element in the second lens group.
In one preferred example of the present invention, the following conditional expression (6) is satisfied:
1.4 less than D1/D3 less than 2.2xe2x80x83xe2x80x83(6) 
where D1 denotes variation in air interval between the first lens group and the second lens group when the state of lens positions is moved from the wide-angle end state to the telephoto end state, and D3 denotes variation in air interval between the third lens group and the fourth lens group when the state of lens positions is moved from the wide-angle end state to the telephoto end state.
In one preferred example of the present invention, the following conditional expression (7) is satisfied:
0.1 less than |f4|/f1 less than 0.3xe2x80x83xe2x80x83(7) 
where f4 denotes the focal length of the fourth lens group.
According to another aspect of the present invention, a variable focal length lens system includes, in order from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a negative refractive power, and an aperture stop located between the first lens group and the fourth lens group. When the state of lens positions is changed from a wide-angle end state to a telephoto end state, all the lens groups are moved to the object side such that the interval between the first lens group and the second lens group increases, the interval between the second lens group and the third lens group decreases, and the interval between the third lens group and the fourth lens group decreases. The second lens group has a negative lens having a concave surface facing to the object side, locating to the most object side of the second lens group, and is moved to the object side when the lens system is focused to a near object. The following conditional expressions (8) and (9) are satisfied:
1.8 less than f1/(fwxc2x7ft)xc2xd less than 3.6xe2x80x83xe2x80x83(8) 
0.15 less than DW23/fw less than 0.25xe2x80x83xe2x80x83(9) 
where f1 denotes the focal length of the first lens group, fw denotes the focal length of the variable focal length lens system in the wide-angle end state, ft denotes the focal length of the variable focal length lens system in the telephoto end state, and DW23 denotes the interval between the most object side lens surface of the second lens group and the aperture stop in the wide-angle end state.
In one preferred example of the present invention, the second lens group consists of a negative lens element having double concave surfaces and a positive lens element having a convex surface facing to the object side. The following conditional expression (10) is satisfied:
1.3 less than (|f2N|+f2P)/fw less than 2.4xe2x80x83xe2x80x83(10) 
where f2N denotes the focal length of the negative lens element in the second lens group, and f2P denotes the focal length of the positive lens element in the second lens group.
In one preferred example of the present invention, the following conditional expression (11) is satisfied:
1.1 less than (|f2|+f3)/fw less than 1.4xe2x80x83xe2x80x83(11) 
where f2 denotes the focal length of the second lens group, and f3 denotes the focal length of the third lens group.
In one preferred example of the present invention, the following conditional expression (12) is satisfied:
0.2 less than |f4|/ft less than 0.4xe2x80x83xe2x80x83(12) 
where f4 denotes the focal length of the fourth lens group.